Development of Tensile Strain Models for Strain Based Design and Assessment
Strain-based design and assessment (SBDA) refers to pipeline design and assessment methods for maintaining pipeline service and integrity under large longitudinal deformation (often defined as longitudinal strains greater than 0.5%). The large longitudinal strains are typically induced by ground movement hazards such as land slide, fault crossing, mining subsidence, frost heave and thaw settlement. The SBDA covers at least two limit states: tensile rupture and compressive buckling.
A joint US government (DOT1 PHMSA2) and industry (PRCI3) project to develop refined tensile strain models was completed in 2011. The work was jointly carried out by Center for Reliable Energy Systems (CRES), C-FER Technologies, and Microalloying International. The model development was led by CRES. The models provide significant improvement to the tensile strain equations in CSA Z662 Annex C (2007) and are applicable for pipe grades from X65 to X100 and two welding processes (i.e., mechanized GMAW and FCAW/SMAW). The tensile strain is given as a function of key material properties and geometry parameters, including pipe wall thickness, girth weld high-low misalignment, pipe strain hardening (Y/T ratio), weld strength mismatch, toughness, girth weld flaw size, and internal pressure.
1DOT: Department of Transportation
2PHMSA: Pipeline and Hazardous Materials Safety Administration
3PRCI: Pipeline Research Council International
Development of Engineering Critical Assessment (ECA) Standards
Pipeline girth welds often contain “imperfections,” which are alternatively termed as “flaws” or “defects.” Traditionally, the tolerable size of those imperfections is set by workmanship-based criteria, such as those in the main body of API Standard 1104. These criteria are empirically-based and historically proven safe in practice. In most cases, they are not quantitatively related to the severity of the defects for the safe operation of the pipelines.
ECA in the context of pipeline girth weld refers to the development of weld imperfection acceptance criteria for the purpose of field girth weld inspection and repair (if needed). The technical basis of ECA is fracture mechanics. When executed correctly, ECA provides a quantifiable level of safety for the project-specific welds and loading conditions. ECA is the preferred method for field girth weld inspection and quality control for long distance pipeline.
CRES staff were the key authors of the ECA based girth weld imperfection acceptance criteria adopted by API 1104 Appendix A. The acceptance criteria were developed under a joint US government (DOT PHMSA) and industry (PRCI) project completed in 2005.
Welding Simulation and Welding Simulation Software
CRES is a leader in the welding simulation of new microalloyed steels and has extensive welding simulation capabilities. Our simulation capabilities cover a wide range of applications, including in-service welding, prediction of heat-affected zone (HAZ) and weld properties, prediction of residual stress and distortion, and heat transfer/fluid flow in welding processes.
CRES has developed numerical models and software tools that combined either simple or sophisticated thermal models with advanced microstructure models. With these models and tools, the mechanical properties of HAZ and welds can be evaluated for welding parameters selection and procedure design. CRES has the expertise to conduct this type of analysis or deliver software that targets a specific welding application.
Welding residual stress and distortion calculation is a computationally intensive multi-physics simulation. CRES staff has been involved in a variety of projects related to residual stress and distortion calculations. We develop special-purpose numerical tools, material constitutive models, and software codes that enable efficient simulation of complex welding processes.
WORK FOR INDUSTRY
Engineering Critical Assessment (ECA) of Girth Welds for New Constructions
For pipeline constructions, ECA in the context of girth welds refers to the development of weld imperfection acceptance criteria for the purpose of field girth weld inspection and repair (if needed). The technical basis of ECA is fracture mechanics.
CRES has worked with contractors and owner companies on inspection and quality control in the fields to ensure welds of consistent quality are produced. The imperfection acceptance criteria are often developed following the specifications of relevant codes and standards.
CRES staff has a long history of developing weld ECA procedures for difficult situations. One of CRES’ unique capabilities is applying fracture mechanics and our welding expertise to difficult situations that are not adequately covered by relevant codes and standards. The examples of such situations are weld high-low misalignment, weld strength undermatching, etc. Furthermore we conduct post-construction weld quality assessment, either before or after hydrotesting and commissioning.
Welding Procedure Qualification
The primary objective of welding procedure qualification is to establish that welds of certain quality can be reliably produced. These welds should have the necessary properties to meet or exceed the requirements set forth by relevant codes, standards, and/or company specifications.
CRES works with pipeline construction contractors and owner companies on welding procedure qualifications. CRES monitors the welding and subsequent mechanical testing to ensure codes and company requirements are met. CRES and contractors and owner companies often collectively work on additional qualification and requirements that may be necessary for the project-specific materials and welding procedures to ensure safety and efficiency.
Fitness for Service Assessment – Girth Weld ECA and Fatigue Assessment
The fitness for service assessment of girth weld defects typically covers both ECA and fatigue assessment.
The ECA procedures specified in codes and standards for new constructions are often used. Moreover, CRES staff has a long history of developing weld ECA procedures for difficult situations that are not adequately covered by relevant codes and standards. The examples of such situations are weld high-low misalignment, weld strength undermatching, wall thickness transitions, extremely large flaws, etc.
CRES has extensive knowledge and experience on welded joint fatigue assessment from fracture mechanics based flaw growth calculation to traditional S-N based analysis (nominal approach, structural stress approach, and local approach). The fatigue assessment is based on fracture mechanics related calculations, stress analyses, fatigue process in materials, and/or with the help of metallurgical examination and metallographic observations tools.
Fitness for Service Assessment – Long and Spiral Seam ECA and Fatigue Assessment
The fitness-for-service procedures specified in codes and standards are mainly for girth welds or long seam welds and not applicable to spiral seam welds.
CRES staff has a long history of conducting fitness-for-service for difficult situations that are not adequately covered by relevant codes and standards. CRES has worked with joined industry clients to conduct fitness-for-service assessment for spiral seam welds and skelp-end welds. The analyses covered both ECA for static loading and fatigue assessment for cyclic loading.
Assessment of Strain Demand and Pipe Integrity under Ground Movement Hazards
Large longitudinal strains can be generated by a number of sources. The most relevant events for onshore pipelines are seismic activities (such as liquefaction, surface faulting, and landslides), mining subsidence, and arctic hazards (such as frost heave and thaw settlement).
The assessment of strain demand and pipe integrity can be a very complex task involving multiple disciplines such as geology, soil mechanics, and soil and pipe interaction. CRES has worked with private industry clients to determine the strain demand and assess the pipe integrity (including both tensile and compressive limit states) for onshore pipelines under various ground movement hazards such as fault crossing and mining subsidence. The strain demand was treated as an integral element in pipe integrity assessment. Our comprehensive approach combining the ground movement with pipe material response ensures consistency between strain demand and strain capacity.
Weld Integrity Assessment for In-Service Pipelines
CRES staff has been involved in the development of in-service welding and weld integrity assessment procedures. In-service welding can be applied to pipeline repair such as the installation of sleeves and direct weld deposition repair. One of the most critical applications of in-service welding is hot-tapping which allows the installation of branch pipes while the run pipe is still in operation under full or partial pressure.
CRES has developed numerical models and software to conduct heat transfer analyses and HAZ microstructure calculation to assist the selection of welding processes and welding parameter. These models and software have been used to assess the propensity of hydrogen induced cracking. As more and more micro-alloyed high strength steels such as X80 and X100 grades are employed in pipeline constructions and other engineering applications, these models and software provide critical information for the propensity of hydrogen induced cracking and HAZ softening.
Development of Testing Procedures
Working with US DOT and private clients, CRES staff has been active in developing material testing techniques with particular focus on the testing of complex weld joints and low-constraint testing, e.g., curved wide plate (CWP), single edge notched bending (SENB), and full-scale pipe tests. Some of our work products have been adopted by international codes and standards.
CRES has provided specimen design and instrumentation plan before the tests and developed procedures for post-processing the test data and extracting fracture toughness parameters.
Lowering-in Stress Analysis for Pipe Construction
Controlling the lowering-in stress during pipeline construction is critical to maintain the girth weld integrity. CRES has worked with private clients to determine the stress induced by the pipe lowering-in and provide recommendations on controlling the lowering-in stress.
Pipe Integrity Assessment for Trenchless Pipe Construction
CRES has worked with private clients to determine the pipe integrity under various trenchless installations including HDD, pipe ramming, direct pipe. The assessment includes stress analysis, ECA, and fatigue assessment.
Fitness for Service Assessment – Dents and Wrinkles
Dents and wrinkles are common threats to pipeline integrity. CRES worked for private clients to provide integrity assessment for dents and wrinkles. The assessment was based on advanced finite element analyses and included both case studies and software tool development.